Polystyrene-glass-bead and styrene/methacrylic acid copolymer-glass-bead composites were investigated by dielectric spectrometry in order to study the Maxwell-Wagner-Sillars (MWS) interfacial relaxation. Untreated glass beads and glass beads with various surface coatings were both used in the composites. The interfacial relaxation is used to test for the influence of the surface coating on the blocking nature of the interface. A good agreement is found between the van Beek calculated values of the maximum loss frequencies and the experimental ones. Through this analysis, the use of a coupling agent is shown to improve the trapping of the ions at the interface between the polymer matrix and the glass inclusions. This was especially the case in styrene-methacrylic acid copolymer filled with glass beads coated with a methacrylic silane. The presence of water causes the maximum loss frequency to increase dramatically and is also thought to improve the blocking of the electric charges at the matrix-inclusions interface.